In the construction of glass-based optical fiber elements, a coating is usually applied to the glass optical fiber immediately after drawing to protect the glass surface from the detrimental effects of chemical and/or mechanical attack which would otherwise occur. Such forms of attack, to which glass optical fibers are particularly susceptible, greatly decrease the mechanical strength of optical fibers and lead to their premature failure.
Conventionally, several coatings are applied to optical fibers, with each serving a specific purpose. A soft coating is applied initially to protect the fiber from microbending losses, and a harder, secondary coating is applied over the soft coating to provide resistance to abrasion.
The connectorization process (i.e., coupling an optical fiber element to another optical fiber element or other optical element via a splicing device or optical connector) conventionally entails the removal of all coating layers such that the bare glass surface is exposed. The glass surface is usually cleaned by wiping it with a soft tissue which has been moistened with an alcohol such as isopropanol. The fiber is then fixed into a connector ferrule or splicing device using an adhesive such as an epoxy, hot melt, or acrylic adhesive. Upon curing (or cooling) of the adhesive, the fiber end face is polished and the connectorization process is complete.
During the connectorization process, the optical fiber is very vulnerable. Initially, the fiber may be nicked by the blades of the tool used to remove the outer coatings during the stripping operation. After stripping, the bare fiber is exposed to elements in the local environment. These are likely to include water vapor and dust particles. Water acts chemically on the surface of the glass and dust acts as an abrasive. Both of these effects contribute to failure of the glass fiber. Most failures in optical fiber systems tend to occur at the sites of connector installation.
One solution to the problem of fiber stripping and exposure during connectorization has been proposed in U.S. Pat. No. 4,973,129. That patent discloses an optical fiber element wherein a resin composition having a Shore D hardness value of 65 or more (specified in the Japanese Industrial Standards at room temperature) is applied to the surface of a glass optical fiber having a numerical aperture (NA) value of 0.35 or more. The resin is then cured to form a primary coating layer which does not have to be peeled from the optical fiber at the time of connectorization. Instead, the primary layer remains on the fiber during connectorization (and thereafter) to prevent the fiber from being damaged as described above. Useable optical fibers are said to be limited to those having a NA value of 0.35 or more because the optical losses caused by microbending ("microbending loss") increase upon covering the optical fiber with such a hard resin. In optical fibers with a NA value below 0.35, microbending loss was found to be so great as to make optical communications impractical. When the NA of the optical fiber is 0.35 or more, however, microbending loss was not found to be a problem.
Unfortunately, optical fiber elements which require an optical fiber having a NA value of 0.35 or more are not commercially useful. As is known, NA is a measure of the angle of light which will be accepted and transmitted in an optical fiber. Optical fiber elements having a NA value of 0.35 or more find limited use in communication, data transmission, and other high bandwidth applications for two reasons: 1) limited information-carrying capacity and 2) incompatibility with existing, standardized communication fibers (which normally have NA values less than 0.29). The information-carrying capacity of an optical fiber is usually expressed as bandwidth. Bandwidth is a measure of the maximum rate at which information can pass through an optical fiber (usually expressed in MHz-km). Bandwidth is inversely proportional to NA because the higher order modes (analogous to higher angles of incident light) have longer paths in the fiber, thereby resulting in pulse broadening or dispersion. The bandwidth limitation of an optical fiber element occurs when individual pulses travelling through that fiber can no longer be distinguished from one another due to dispersion. Thus, the larger the NA value of an optical fiber, the smaller is that fiber's bandwidth (and therefore information-carrying capacity). Most commercially useful optical fibers have a NA value of 0.29 or less. As compared to the information-carrying capacity of such commercially useful optical fibers, fibers having a NA value of 0.35 or more carry far less information in a given period of time and are therefore undesirable.
Incompatibility becomes a problem when one optical fiber element is spliced or connected to another optical fiber element. In this instance, it is important to minimize signal attenuation at the point of connection. When an optical fiber element with a higher NA value is spliced to a fiber with a lower NA value, all light exceeding the NA value of the receiving fiber will be attenuated. Light-carrying capacity is proportional to the square of the NA. Thus, as an example, 38% of the light will be lost when transmitted from a fiber with a NA value of 0.35 to a fiber with a NA value of 0.275. This is a significant and unacceptable loss in signal.
Accordingly, a need exists in the art for an optical fiber element which protects the optical fiber during connectorization and which allows the use of optical fibers having NA values smaller than 0.35.